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Tue, 08 Apr 2025 09:50:37 -0400 X-MC-Unique: JpD0Exd5NUGfZXnjMRKrmA-1 X-Mimecast-MFC-AGG-ID: JpD0Exd5NUGfZXnjMRKrmA_1744120236 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8BF4C1956064; Tue, 8 Apr 2025 13:50:36 +0000 (UTC) Received: from fedora (unknown [10.72.120.20]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 33A7A1956094; Tue, 8 Apr 2025 13:50:31 +0000 (UTC) Date: Tue, 8 Apr 2025 21:50:26 +0800 From: Ming Lei To: Nilay Shroff Cc: Christoph Hellwig , Jens Axboe , linux-block@vger.kernel.org, syzbot+4c7e0f9b94ad65811efb@syzkaller.appspotmail.com Subject: Re: [PATCH] block: don't grab elevator lock during queue initialization Message-ID: References: <20250403105402.1334206-1-ming.lei@redhat.com> <20250404091037.GB12163@lst.de> <92feba7e-84fc-4668-92c3-aba4e8320559@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 On Tue, Apr 08, 2025 at 06:55:26PM +0530, Nilay Shroff wrote: > > > On 4/8/25 1:08 PM, Ming Lei wrote: > > On Mon, Apr 07, 2025 at 01:59:48PM +0530, Nilay Shroff wrote: > >> > >> > >> On 4/7/25 8:39 AM, Ming Lei wrote: > >>> On Sat, Apr 05, 2025 at 07:44:19PM +0530, Nilay Shroff wrote: > >>>> > >>>> > >>>> On 4/4/25 2:40 PM, Christoph Hellwig wrote: > >>>>> On Thu, Apr 03, 2025 at 06:54:02PM +0800, Ming Lei wrote: > >>>>>> Fixes the following lockdep warning: > >>>>> > >>>>> Please spell the actual dependency out here, links are not permanent > >>>>> and also not readable for any offline reading of the commit logs. > >>>>> > >>>>>> +static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, > >>>>>> + struct request_queue *q, bool lock) > >>>>>> +{ > >>>>>> + if (lock) { > >>>>> > >>>>> bool lock(ed) arguments are an anti-pattern, and regularly get Linus > >>>>> screaming at you (in this case even for the right reason :)) > >>>>> > >>>>>> + /* protect against switching io scheduler */ > >>>>>> + mutex_lock(&q->elevator_lock); > >>>>>> + __blk_mq_realloc_hw_ctxs(set, q); > >>>>>> + mutex_unlock(&q->elevator_lock); > >>>>>> + } else { > >>>>>> + __blk_mq_realloc_hw_ctxs(set, q); > >>>>>> + } > >>>>> > >>>>> I think the problem here is again that because of all the other > >>>>> dependencies elevator_lock really needs to be per-set instead of > >>>>> per-queue which will allows us to have much saner locking hierarchies. > >>>>> > >>>> I believe you meant here q->tag_set->elevator_lock? > >>> > >>> I don't know what locks you are planning to invent. > >>> > >>> For set->tag_list_lock, it has been very fragile: > >>> > >>> blk_mq_update_nr_hw_queues > >>> set->tag_list_lock > >>> freeze_queue > >>> > >>> If IO failure happens when waiting in above freeze_queue(), the nvme error > >>> handling can't provide forward progress any more, because the error > >>> handling code path requires set->tag_list_lock. > >> > >> I think you're referring here nvme_quiesce_io_queues and nvme_unquiesce_io_queues > > > > Yes. > > > >> which is called in nvme error handling path. If yes then I believe this function > >> could be easily modified so that it doesn't require ->tag_list_lock. > > > > Not sure it is easily, ->tag_list_lock is exactly for protecting the list of "set->tag_list". > > > Please see this, here nvme_quiesce_io_queues doen't require ->tag_list_lock: > > diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c > index 777db89fdaa7..002d2fd20e0c 100644 > --- a/drivers/nvme/host/core.c > +++ b/drivers/nvme/host/core.c > @@ -5010,10 +5010,19 @@ void nvme_quiesce_io_queues(struct nvme_ctrl *ctrl) > { > if (!ctrl->tagset) > return; > - if (!test_and_set_bit(NVME_CTRL_STOPPED, &ctrl->flags)) > - blk_mq_quiesce_tagset(ctrl->tagset); > - else > - blk_mq_wait_quiesce_done(ctrl->tagset); > + if (!test_and_set_bit(NVME_CTRL_STOPPED, &ctrl->flags)) { > + struct nvme_ns *ns; > + int srcu_idx; > + > + srcu_idx = srcu_read_lock(&ctrl->srcu); > + list_for_each_entry_srcu(ns, &ctrl->namespaces, list, > + srcu_read_lock_held(&ctrl->srcu)) { > + if (!blk_queue_skip_tagset_quiesce(ns->queue)) > + blk_mq_quiesce_queue_nowait(ns->queue); > + } > + srcu_read_unlock(&ctrl->srcu, srcu_idx); > + } > + blk_mq_wait_quiesce_done(ctrl->tagset); > } > EXPORT_SYMBOL_GPL(nvme_quiesce_io_queues); > > Here we iterate through ctrl->namespaces instead of relying on tag_list > and so we don't need to acquire ->tag_list_lock. How can you make sure all NSs are covered in this way? RCU/SRCU can't provide such kind of guarantee. > > > And the same list is iterated in blk_mq_update_nr_hw_queues() too. > > > >> > >>> > >>> So all queues should be frozen first before calling blk_mq_update_nr_hw_queues, > >>> fortunately that is what nvme is doing. > >>> > >>> > >>>> If yes then it means that we should be able to grab ->elevator_lock > >>>> before freezing the queue in __blk_mq_update_nr_hw_queues and so locking > >>>> order should be in each code path, > >>>> > >>>> __blk_mq_update_nr_hw_queues > >>>> ->elevator_lock > >>>> ->freeze_lock > >>> > >>> Now tagset->elevator_lock depends on set->tag_list_lock, and this way > >>> just make things worse. Why can't we disable elevator switch during > >>> updating nr_hw_queues? > >>> > >> I couldn't quite understand this. As we already first disable the elevator > >> before updating sw to hw queue mapping in __blk_mq_update_nr_hw_queues(). > >> Once mapping is successful we switch back the elevator. > > > > Yes, but user still may switch elevator from none to others during the > > period, right? > > > Yes correct, that's possible. So your suggestion was to disable elevator > update while we're running __blk_mq_update_nr_hw_queues? And that way user > couldn't update elevator through sysfs (elv_iosched_store) while we update > nr_hw_queues? If this is true then still how could it help solve lockdep > splat? Then why do you think per-set lock can solve the lockdep splat? __blk_mq_update_nr_hw_queues is the only chance for tagset wide queues involved wrt. switching elevator. If elevator switching is not allowed when __blk_mq_update_nr_hw_queues() is started, why do we need per-set lock? Thanks, Ming